The present invention pertains to the field of beam blankers. More particularly, this invention relates to a one dimensional beam blanker array.
An electron beam can be controlled with exacting precision, which makes electron beams particularly useful for a number of applications. A scanning electron microscope (SEM) is a common example. As an electron beam is scanned across a surface, secondary electrons are emitted from the surface, detected, and used to create a topographical image of the surface. The precision of the electron beam allows a SEM to detect incredibly minute detail.
As another example, in addition to inspecting a surface, an electron beam can be used to imprint an image on a surface. This is often called electron beam lithography, and it can be done in many different ways. In a typical process, a surface is coated with a film that is sensitive to electrons. An electron beam exposes only certain areas of the film and a chemical process dissolves away either the exposed or unexposed film, leaving behind an image imprinted on the surface.
Lithography has a number of useful applications, not the least of which is integrated circuit (IC) manufacturing. Circuit designs can be built on a chip by imprinting one or more layers of circuit components on the chip""s surface. Electron beam lithography performed directly on a wafer, however, has not been commercially successful because the primary competing technology, optical lithography using photomasks, has been much faster.
Optical lithography uses a light source and a mask to imprint an image onto a surface. Typically, the surface is covered with a photosensitive film, and the light source partially exposes the film by projecting an image through the mask onto the surface. Chemical processing removes either the exposed or unexposed film to leave the image imprinted on the surface. Optical lithography has been much faster than electron beam lithography largely because, in optical lithography, an entire image can be imprinted in a single exposure where as, in electron beam lithography, the image is sequentially xe2x80x9cdrawnxe2x80x9d using the electron beam.
Although optical lithography has been much faster than electron beam lithography, optical lithography is rapidly approaching its theoretical limits. That is, progress is continually pushing for smaller and denser designs. For integrated circuits in particular, smaller and denser designs translate into smaller chips and/or more functionality per chip. As minimum feature sizes in these designs drop into the submicron range, features are often smaller than the wavelength of the light sources used in optical lithography. At sub-wavelength levels, the behavior of light changes and becomes more complicated. A variety of technologies have been developed to compensate for the sub-wavelength behavior of light in optical lithography, including mask manipulations and multiple exposure techniques. Each technology, however, adds cost and time to the manufacturing process. At some point, progress will necessitate feature sizes that optical lithography simply cannot create, or cannot create economically.
In order to progress further, electron beam lithography is likely to be the way of the future. With the precision control of an electron beam, electron beam lithography can create incredibly small feature sizes, well into the nanometer range, without using a mask. Speed, however, remains a primary stumbling block for commercially viable electron beam lithography.
There are two major obstacles to speed in electron beam lithography. The first is Coulomb interaction, where like-charged particles repel one another. For instance, assuming that a certain dose of electrons are needed to expose a film, and assuming a beam has a particular dose rate for delivering electrons, the speed at which the beam moves across the film is limited. If the beam goes too fast, the film will not be adequately exposed. One way to increase the speed is to increase the dose rate of the beam. With more electrons being deposited per unit time, the beam can move faster and still adequately expose the film.
There is a trade-off, however, due to Coulomb interaction. The negatively charged electrons in an electron beam repel one another, causing the beam to spread out as the beam travels to the film. Increasing the number of electrons causes more spreading in the beam, making the beam size fatter and less precise on the film. In other words, due to Coulomb interaction, speed is limited by a combination of the film""s exposure sensitivity, the desired size of the beam, and the dose rate of the beam.
The second major obstacle in electron beam lithography is the pattern delivery rate. Various approaches have been developed to deliver patterns with an electron beam. One main approach is raster scan, in which an entire surface is scanned, usually going horizontally across the surface and moving vertically down the surface one horizontal line at a time until every xe2x80x9cpixelxe2x80x9d of the surface is scanned. For a single-beam system, the beam is turned on wherever the pattern indicates that the film is to be exposed.
Another main approach is vector scan, in which a surface is broken up into vector locations. Each vector location corresponds to a small block of surface area. To project an image in a single-beam system, the beam is directed to a vector location on the surface that includes part of the pattern and the beam flashes small primitive shapes of electrons to build up the desired pattern.
Vector scan can be much faster than raster scan for images that do not cover large portions of a surface area because vector scan does not need to scan over every pixel of the surface. However, most patterns, particularly in IC designs, use nearly half of available surface area. In which case, vector scan can actually be slower than raster scan.
In both of these approaches, the pattern delivery rate is limited by the physical constraints of the system. Often times, electric fields are used to scan an electron beam over a scan field and mechanical stage motion is used to move from one scan field to another. For instance, in raster scan, a wafer can be attached to a stage that can move relative to an electron beam column. The stage can align the beam with a particular scan field on the wafer and electric fields can sweep the beam across the scan field. Similarly, in vector scan, the vector location can be addressed by moving the stage and electric fields can be used to imprint the shapes.
Using a single beam, both raster scan and vector scan can take many hours to write a typical IC pattern to a wafer, compared to several minutes for optical lithography. In order to increase speed, a number of research programs have attempted to create multiple-beam systems. One of these systems uses multiple single-beam mini-columns in close proximity. While some progress has been made in miniaturizing single-beam columns, this approach has been largely unsuccessful at least partially due to the vast number of columns needed. To be competitive with optical lithography, an electron beam system is likely to need many thousands of columns. That level of integration has proven very difficult to attain.
Another one of these systems generates multiple beams from a single beam column using a micro-blanker array. A blanker is a common element in most electron beam columns. An electron source produces a stream of electrons. A series of electrical elements generate electric and/or magnetic fields that direct the stream of electrons into a beam. The beam passes through a blanker that is aligned with an aperture. The blanker usually comprises two electrodes. When a voltage is applied across the electrodes, the resulting electric field diverts the beam away from the aperture, preventing the beam from passing through the remainder of the column. By modulating the blanker voltage, the beam can effectively be turned on and off at a much faster rate than the electron source can be turned on and off.
Rather than a single blanker, a micro-blanker array usually includes multiple holes in an IC chip, with electrodes and control circuitry integrated into the chip around each hole. Segments of the beam passing through holes in the array can be individually diverted away from the aperture by separately applying a voltage to the electrodes surrounding respective holes, thus creating multiple beam segments from a single beam source. With the micro-blanker array, the beam can cover a block of surface area at a time rather than one dot at a time.
As with column miniaturization however, the micro-blanker array has been largely unsuccessful due to numerous limitations and problems in the technology. For instance, the supporting circuitry around each hole requires a certain amount of space and separation from neighboring holes, and more space is needed for larger numbers of holes. The spacing requirement limits how close the holes can be and therefore limits the fraction of the beam current that can be projected through the blanker array.
Furthermore, in order to divert electrons, each hole requires a certain combination of electrode length and voltage level. The shorter the length of the electrodes, the higher the voltage level that is needed. The length of the electrodes is limited by the thickness of the IC chip, necessitating a certain minimum voltage level. The higher the voltage level, the more isolation and separation that is needed between holes in the array and the less likely it is to find a semiconductor process capable of providing the necessary voltages.
As a result of these and countless other problems, most multiple-beam lithography programs have been unsuccessful.